PARTICLES HAVING A SINTERABLE CORE AND A POLYMERIC COATING, USE THEREOF, AND ADDITIVE MANUFACTURING METHOD USING THE SAME

Description

Introduction

[0001] In today's industries the fabrication of complicated geometrical shapes is typically made with the aid of computer aided design, (CAD). With Solid Freeform Fabrication, (SFF) also called rapid prototyping, (RP) one can utilize CAD models to create physical objects through a layer by layer technique, where a highly intricate 3D model becomes segmented into thin 2D slices and thereby feasible to be constructed with the aid of 3D printers.

[0002] Manufacturing by 3D printing is one step closer to having finished goods directly from digital data, eliminating time staking and costly tooling. Since it is an additive manufacturing process it does not generate waste as is common in subtractive machining. Various objects can be manufactured from the same stock of powder, little labor and skills are required for multi-part designs, and full creation by digital information reduces risk for human error.

[0003] Today, many different manufacturing methods are employed, including Fused Deposition Modelling (FDM), Direct Ink Writing (DIW), Inkjet 3D Printing, Selective Laser Sintering (SLS), Electron Beam Melting (EBM), Selective Laser Melting (SLM), Laminated Object Manufacturing (LOM), Directed Energy Deposition (DED) and Electron Beam Free Form Fabrication (EBF). Each method has its own advantages and disadvantages, in particular with regard to the obtained resolution, the materials that can be employed, manufacturing time, equipment costs, etc.

[0004] 3D printing takes on where other manufacturing processes fail to deliver. As a layer-by-layer method it is the main technique to successfully produce complex shaped components from a sinterable material, such as metal powder. A particular type of 3D printing is a Powder Bed and Inkjet head 3D printing method (Inkjet printing on a powder bed, hereinafter referred to as 3DP).

[0005] In this method, a powder layer is spread over a build platform. The powder is then selectively covered by a liquid binder composition (also named "ink"), which is ejected from an inkjet head and contains a binder component, which is often a polymer. Typically, heat is applied on the platform in order to evaporate the solvent (typically water or another low temperature-boiling solvent, such as methanol, ethanol, or acetone) in the ink, leaving almost only the binder component. A new powder layer is applied, and the process is repeated. The placement of the binder/ink determines the final geometrical shape of the component, where additionally shrinkage needs to be considered when sintering of the component is needed. When the printing step is finished a drying/curing step is performed in order to evaporate the solvent and let the 3D-printed parts harden into said shape. This process is illustrated in Figure 1.

[0006] Subsequently, excess particles (not bound by the "ink") are removed, e.g. by blowing air or careful removal by hand using a soft brush or similar, and the so-called "green part" is obtained. This green part is then subjected to a step of removing the binder component (so-called "debinding"), which is typically effected by heat treatment leading to thermal decomposition or evaporation of the binder component, leaving the so-called "brown part". Other treatments may also be contemplated, such as catalytic removal or solvent extraction, but removal by heat treatment is preferred. Generally, this is followed by a sintering step in order to fuse the particles' boundaries, giving the final part. This method can be applied to form full-metal parts made from e.g. stainless steel. This technology is already put to market and used for the manufacture of consumer goods, e.g. by Digital Metal AB.

[0007] When it comes to printing with the 3DP technique, the stability of the green part is critical when producing delicate structures. This is due to the fact that for the debinding treatment, the green part must be removed from the 3D printer unit and must be put in e.g. an oven in order to effect thermal debinding. Hence, a high Green Strength (strength of the green part) is required for certain structures.

[0008] To make 3DP a better competitive alternative to for example the SLS technique, the Green Strength needs to be increased in order to push the dimension limit to even more delicate structures than what is possible today.

[0009] Further, it is desired than an improvement in Green Strength can be obtained without requiring changes to the actual manufacturing process, which is well established. This implies, as one example, that the viscosity of the binder composition/ink can remain unaltered, as an increase or decrease in viscosity are expected to cause problems with currently used inkjet heads and may impair product quality.

Problem to be Solved

[0010] It is an object of the present invention to provide means for increasing the Green Strength of a green part obtained during a 3DP process.

[0011] It is a further object of the present invention to provide particles suitable for use in a 3DP process that allow increasing the Green Strength beyond the Green Strength currently obtainable, preferably without requiring further adaptations to the 3DP process.

[0012] It is a further object of the present invention to provide particles suitable for use in a 3DP process that allow adjusting the Green Strength of the green part by increasing the amount of binder composition/ink, thereby increasing the versatility of the particles.

[0013] It is a further object of the present invention to provide particles suitable for use in a 3DP method that have protection against outer influences and are less prone to surface reactions such as oxidation, without the need for deprotection steps prior to the use of the particles.

[0014] It is a further object of the present invention to provide a 3DP manufacturing process allowing to obtain a higher Green Strength of the green part while still allowing to obtain a good quality final product.

[0015] In the art, aspects of the above problems to be solved have been addressed, in particular in the following documents:
WO 2016/068899 A1, wherein according to the abstract is taught: "A three-dimensional (3D) printing method, a build material (consisting of an inorganic particle and a polymer attached thereto) is applied. The polymer is a continuous coating having a thickness from about 3 nm to about 1500 nm, or nano-beads having an average diameter from about 3 nm to about 1500 nm."

[0016] US 6048954, wherein according to the abstract is taught: "The development of polymer binder compositions that provide novel binders for high temperature inorganic particulates, especially metal and ceramic particulates is described. These materials are especially useful in a laser beam sintering process known as SLSTM that forms accurately shaped high strength green objects."

[0017] US 5749041, wherein according to the abstract is taught: "A method of fabricating articles, such as prototype parts and prototype tooling for injection molding. is disclosed. The method begins with the fabrication of the article in a "green" state by the selective laser sintering, or another additive thermal process, applied to a composite powder. Preferably a powder of metal particles coated with a thermoplastic polymer."

[0018] In Nelson et al., Industrial & Eng. Chem. Res. AMC, v34, n5 pp 1641-1651, 1995, there is taught in accordance with the title: "Selective sintering of polymer coated silicon carbide powders." In Lakshminarayan et al. Adv. Powder Metall. v. 3, pp77-85, 1995, there is taught in accordance with the title: "Manufacture of iron-copper parts using selective laser sintering".

[0019] In WO 2017007011 there is taught in accordance with the abstract: "Provision of a metal fine particle-containing composition which enables metal parts on electrical components to be joined at a relatively low temperature, and which achieves a strong joint and leaves little residual organic matter after joining. The metal fine particle-containing composition contains: metal fine particles (P1) comprising a metal element (M) having a bulk melting point of above 420°C, the surface of the particle being at least partially or wholly covered by a coating substance (C), and the primary particle diameter being 1 to 500nm; a low melting point metal powder (P2) comprising a metal or an alloy having a bulk melting point of 420°C or lower; and an activating agent (A) which decomposes and removes the coating substance (C) from the surface of the metal fine particle (P1)."

Summary of the Disclosure

[0020] The present invention aims at solving some or all of the above objects and provides particles according to claim 1. Moreover, a use of the particles according to claim 4, a green part according to claim 6 and an additive manufacturing method according to claim 9 are provided.

[0021] The invention is detailed herein in accordance with the claims as granted.

Brief Description of the Drawings

[0022] 

Figure 1 shows a schematic representation of a 3DP method. Herein, an inkjet 1 obtains the Wetting Ink / Binder composition (solution) from a reservoir and selectively applies the binder composition / Wetting Ink to a powder bed 2 provided on a build platform 3. Thereby an object / part 4 is formed, wherein the particles of the powder bed are preliminarily bound together by the Wetting Ink.

Figure 2 shows a schematic representation of the experimental setup for determining the Green Strength in a 3-point bending test. Herein, h represents the height of the sample, P represents the force, S represents the distance between the support rollers, and L denotes the length of the test specimen (corresponding to the width for a square test specimen).

Definitions

[0023] The following terms will be used in the following detailed description:
The term "polymer" and "polymeric compound" are used synonymously. A polymer or polymeric compound is generally characterized by comprising 5 or more, typically 10 or more repeating units derived from the same monomeric compound/monomer. A polymer or polymeric material generally has a molecular weight of at least 300, typically 1000 or greater. The polymer may be a homopolymer, a random copolymer, a block copolymer or a mixture of any of these, unless reference is made to specific forms thereof. The polymer may be synthesized by any method known in the art, including radical polymerization, cationic polymerization and anionic polymerization.

[0024] A monomer in the sense of the present invention is typically a molecule of a chemical species that is capable to react with another molecule of the same chemical species to form a dimer, which then is able to react with a another molecule of the same chemical species to form a trimer, etc., to ultimately form a chain wherein 5 or more, preferably 10 or more repeating units derived from the same chemical species are connected to form a polymer. The group of the monomer molecule capable of reacting with a group of another monomer molecule to form the polymer chain is not particular limited, and examples include an ethylenically unsaturated group, an epoxy group, etc. The monomer may be monofunctional, bifunctional, trifunctional or of higher functionality. Examples of bifunctional monomers include di(meth)acrylates and compounds possessing both a carboxylic acid group and an amide group, and examples of trifunctional monomers include tri(meth)acrylates.

[0025] The term "poly(meth)acrylate" is used to jointly denote polymers derived from methacrylic acid, acrylic acid and/or their esters, such as methyl methacrylate or butyl acrylate. The ester residue is preferably a hydrocarbon group having 1 to 20 carbon atoms, more preferably an alkyl group.

[0026] The term "weight average molecular weight" denotes the weight average molecular weight determined by a GPC method using polystyrene as standard.

[0027] In the present invention, all physical parameters and properties are measured at room temperature (20 °C) and at atmospheric pressure (10⁵Pa), unless indicated differently. Further, all values given in % generally relate to % by weight, unless indicated otherwise. Whenever reference is made to a characteristic or property that needs to be determined by a specific test method, the methods mentioned in the Examples can be used. This applies in particular to the measurement of the Green Strength, the amount of polymeric coating, and the particle size.

[0028] The term "sinterable" is used to denote inorganic materials that have a melting point of 450°C or higher, preferably 500 °C or higher, more preferably 600 °C or higher. Sinterable materials in this sense include metals, alloys, ceramics, and glasses having the required melting point. For composites (such as cermet), it would be sufficient if at least some of the material present on the outside of the particle has a melting temperature in the above range, so that the particles may bind to each other during the sintering treatment to form the final sintered body.

[0029] As used herein, the indefinite article "a" indicates one as well as more than one and does not necessarily limit its reference noun to the singular, unless this is evident from the context.

[0030] The term and/or means that either all or only one of the elements indicated is present. For instance, "a and/or b" denotes "only a", or "only b", or "a and b together". In the case of "only a" the term also covers the possibility that b is absent, i.e. "only a, but not b".

[0031] The terms "comprising" and "containing" as used herein are intended to be non-exclusive and open-ended. A composition comprising or containing certain components thus may comprise other components besides the ones listed. However, the term also includes the more restrictive meanings "consisting of" and "consisting essentially of". The term "consisting essentially of" allows for the presence of up to and including 10 weight%, preferably up to and including 5% of materials other than those listed for the respective composition, which other materials may also be completely absent.

[0032] The term "Green Strength" used in the present invention relates to the Green Strength of a rectangular test specimen taken from a green part, determined according to the method specified in the Examples section.

Detailed Description of the Disclosure and the Invention

[0033] The present invention is based on the finding that providing a polymeric coating on a part or the entirety of the surface of a sinterable core allows increasing the Green Strength of a green part by increasing the amount of Wetting Ink / liquid binder composition applied during the manufacturing process of the green part. This allows, on the one hand, increasing the Green Strength to levels that could not be obtained with non-coated sinterable particles so far, and, on the other hand, provides particles with which the Strength of a green body can be adjusted simply by varying the amount of binder composition / Wetting Ink. As such, the particles of the present invention are more versatile in this respect as prior art, uncoated particles. Further, the coating may provide at least partial protection against outer influences and surface reactions.

[0034] The present invention thus overcomes shortcomings in the prior art, and provides a new way of improving / modifying the Green Strength of a green part. Without the need for changing the actual process for producing the green part. This is favorable from both a technical as well as economic point of view, as existing equipment can be used without any adjustments. Further, existing processes do not need to be adapted, provided adaptation was at all technically feasible.

[0035] In this respect, although there have been prior attempts to improve the Green Strength of a green part, the prior art did not succeed in providing such particles and solving the problem of providing means for adjusting / increasing the Green Strength of a green part. Notably, increasing the concentration of polymeric binder in the "ink" was previously not possible, as this would lead to an increase in the viscosity of the binder composition / Wetting Ink to be ejected from the print head, which in turn leads to problems with the inkjet head and impairs the accuracy of the ink deposition, whereas on the other hand the amount of ink could not simply be increased, as higher amounts of binder composition / Wetting Ink could not be absorbed by the sinterable particles.

[0036] Further, as is demonstrated in the Examples and Comparative Examples, simply increasing the amount of ink for non-coated particles does not at all lead to an increase in Green Strength, but rather leads to a significant reduction in Green Strength. This problem is solved by the present invention, wherein the sinterable particles are coated fully or at least in part by a polymeric coating composition prior to the use in a 3D manufacturing method such as 3DP where additional binder composition/ink is applied from a print head (see Figure 1).

[0037] The aspects and materials used in the present invention will now be described in more detail:

Sinterable Core

[0038] The particles of the present disclosure have a sinterable core on which a polymeric coating is applied on at least a part of surface of the core.

[0039] In accordance with the invention, the sinterable core is a stainless steel core and the polymeric coating is at least 80% by weight of the polymeric coating of polyvinyl pyrrolidone.

[0040] Herein, "made of" describes that the particles consist of stainless steel. Unavoidable impurities may however be present. As such, 95% by weight or more of the core of the sinterable particles consists of stainless steel, with the remainder being unavoidable impurities. Preferably, at least 98% by weight or more, and more preferable at least 99% by weight or more of the core of the sinterable particles is formed by the metal, metal alloy, glass, ceramic material or a mixture thereof.

[0041] The sinterable particles may be of any shape, but spherical particles are preferable. This is due to the fact that spherical particles have good flow characteristic and offer high packing density benefiting the strength of the final product.

[0042] The particles of the present disclosure have a polymeric coating on at least a part of the surface of the sinterable core. The coating contains 80% by weight or more, more preferably 90% by weight or more, of a polymer, relative to the total weight of the polymeric coating. In one embodiment, the polymeric coating essentially consists of or consists of a polymer.

[0043] The polymeric coating is present in an amount of 0.10 to 10.00 % by weight, relative to the total weight of the particles. If the amount is less than 0.10 %, it has been found that no significant effect on the capability to modify the Green Strength by increasing the amount of liquid binder can be obtained. If the amount is higher than 10.00% by weight, the shrinkage during the subsequent sintering tends to become large, which might impair the product quality in particular with regard to strength and yield stress. Overall, a high amount of polymeric coating is economically and environmentally unfavorable.

[0044] The lower amount of the polymeric coating is 0.10% by weight, but more prominent effects are obtained if the amount is 0.30% by weight or more or 0.50% by weight or more. Thus, preferably, the lower limit of the amount of a polymeric coating is 0.30% by weight or more, more preferably 0.50% by weight or more.

[0045] The upper limit of the amount of the polymeric coating is 10.00% by weight, relative to the total weight of the particles. Yet, in order to reduce the influence on shrinkage, it is preferable that the amount is 5.00% by weight or less, more preferably 3.00% by weight or less, further preferably 2.00% by weight or less, still more preferably 1.50% by weight or less.

[0046] The polymer present in the polymeric coating according to the disclosure is polyvinyl pyrrolidone. It has been found that a relatively low weight average molecular weight (Mw) is beneficial for obtaining the desired balance of properties, such as good adhesiveness and good removability by solvent extraction or heat treatment. As such, the weight average molecular weight of the polymer is between 1,000 and 50,000, with a molecular weight of 5,000 to 30,000 being preferred. Polyvinyl pyrrolidone (PVP) having a molecular weight of 1,000 to 50,000, more preferably 2,500 to 40,000, further preferably 5,000 to 30,000, is the polymer for providing the polymeric coating on the surface of the sinterable stainless steel core of the particles of the present invention.

[0047] The polymeric coating according to the disclosure and the invention may essentially consist of the polymer, as outlined above. However, it is also possible that additives are present in the polymeric coating. In particular the presence of a surfactant or wetting agent can be beneficial, as this is believed to be able to increase the amount of "ink" that can be absorbed during the production of the green part, which in turn is believed to increase the Green Strength of the green body. The surfactant can be any of anionic, cationic and non-ionic, but is preferably non-ionic. The non-ionic surfactant is preferably a compound or polymer having a (weight average) molecular weight Mw of 1,000 or less, preferably 500 or less, such as Tego Wet 500 used in the Examples, as well as alkylene oxide surfactants of the Pluronic^™ series (BASF). The wetting agent is preferably a polyol, such as a sugar alcohol such as sorbitol, or another polyol such as glycerol, ethylene glycol or propylene glycol.

[0048] The particle size of the particles of the present invention is not particularly limited, but needs to be suitable for an additive manufacturing process. As such, the particle size is preferably such that 95% by weight or more of the particles have a diameter of 100 µm or less, preferably 72 µm or less, further preferably 50 µm or less, as determined by a light scattering method described in more detail in the Examples section.

Use of the Particles and Manufacturing Method

[0049] During the use of the particles for forming a green part, a liquid binder composition ("ink") is used to preliminary bind the particles. The liquid binder composition typically comprises a polymer selected from the same group of polymers as recited above for the polymer in the polymeric coating of the particles of the present invention. Preferably, the polymer present in the liquid binder composition ("ink") is of the same type as the polymer present in the polymeric coating, although this is not a necessity and the polymers may be of different types. An embodiment in accordance with the invention wherein the polymers are of the same type would for instance be one wherein both the polymer in the polymeric coating and the polymer present in the ink are polyvinyl pyrrolidone, each preferably having a Mw in the range of 1,000 - 50,000, more preferably 5,000 - 30,000. This embodiment can be applied to all kinds of sinterable cores according to the disclosure, e.g. those made of stainless steel according to the invention.

[0050] Once the green part has been formed by removing excess powder that is not bound by the ink, it is generally necessary to remove the binder composition together with the polymeric coating by a so-called "debinding" treatment. This step is as such known in the art and can be effected by heat treatment, by catalytic decomposition of the polymer in the polymeric coating and the ink (e.g. by using an acid in case of acid-decomposable polymers), or by solvent extraction. For this reason, it is generally preferable if the polymer present in the polymeric coating and/or the ink is water-soluble or soluble in solvents that are easily removable due to evaporation, such as methanol, ethanol or acetone. Water-soluble polymers are particularly preferred, also in view of the manufacturing process, where water-based "inks" and heating are conducted. As such, preferably one or both of the polymers present in the polymeric coating and/or the "ink" are water-soluble.

[0051] The debinding can be conducted thermally. Here, the green part is preferably heated to a temperature in the range of 250 to 500°C. Usually, performing a thermal debinding treatment for 3 to 10 hours is sufficient for removing the polymer and to form the brown part. A good debinding can generally be obtained within 6 - 8 hours.

[0052] Subsequent to the formation of the brown part, it is often desired to sinter the resulting brown part in order to fuse the sinterable particles at their surfaces, to provide strength and integrity to the resulting product. This is typically conducted by slowly heating (e.g. at a heating range of 1 to 5°C/minute) the brown part to a temperature of about 1,000 to 1,500°C for 10 to 20 hours, followed by cooling at moderate cooling rates (15°C/minute or less).

[0053] The present invention will be demonstrated in more detail by way of the following Examples. These are however not intended to limit the scope of the present invention in any way, and the protective scope of the present application is determined solely by the appended claims.

EXAMPLES

Examples 1 to 9 and Comparative Examples 1 - 3

Preparation of Particles A

[0054] 990 g of gas atomized stainless steel particles 316L (obtainable from Carpenter under the tradename CarTech^® 316L with the nominal composition, in wt.-%, 0.03 C, 2.00 Mn, 0.045 P, 0.030 S, 1.00 Si, 16.00 - 18.00 Cr, 10.00 - 14.00 Ni, 2.00 - 3.00 Mo, balance Fe) were put in a mixing chamber and covered with a lid having an opening for the shaft of an overhead stirrer and an additional opening allowing the addition of liquid.

[0055] Separately, a Coating Solution A was prepared by adding 7.5 g of polyvinyl pyrrolidone (PVP) having a weight average molecular weight Mw of 25,000 to 50 ml of a solution of 74.65% water, 12 % triethylene glycol, 5 % 1,2 hexane diol, 3.25 of nonionic surfactant, 5% of PVP with an Mw of 25,000 - 30,000 and 0.1 % of a Cyan dye. The resulting mixture was stirred to dissolve the PVP in the solution.

[0056] After starting the overhead stirrer, the entire Coating Solution A was slowly added via the opening in the lid of the mixing chamber. After stirring for about 5 minutes, the resulting particles to which the Coating Solution A had been added were transferred to a crucible which was then put into an oven. The particles were dried at 200°C for 3 hours to obtain particles having a sinterable core made of stainless steel which were covered at least in part by a polymeric coating. The dried particles were then ground in order to break up any agglomerates that may have formed. Subsequently, the particles were sieved in order to remove any particles having a size exceeding 71 um.

[0057] Since the Coating Solution A contained 10 g of PVP and the amount of stainless steel particles was 990 g, the aimed amount of PVP was 1% by weight, relative to the total of the particles. Due to losses by transfer to the crucible and liquid remaining on the side walls of the crucible and/or the mixing apparatus, the actual PVP content was lower and determined to 0.68 wt.-% (see Table 1).

Preparation of Particles A'

[0058] Particles A' were prepared in the same manner as particles A, except for pre-drying the particles obtained after addition of Coating Solution A for 2 hours at 150°C before drying at 200°C for 3 hours.

[0059] The resulting particles A' had a lower amount of polymeric coating of 0.51 wt.-% as compared to Particles A, which can be explained by a higher amount of evaporated components of the Coating Solution A during the pre-drying as compared to Particles A.

Preparation of Particles B

[0060] Particles B were produced in the same manner as Particles A, except for using the Coating Solution B instead of the Coating Solution A. Coating Solution B is a solution obtained by adding 7.5 g of PVP (Mw=25,000) to 50 ml of a solution of 84.4% water, 10 % ethylene glycol, 5% of PVP with an Mw of 15,000, 0.5 % of Tego Wet^™ 500 (nonionic surfactant, Oxirane 2-methyl-, polymer with oxirane, mono(3,5,5-trimethylhexyl) ether, CAS 204336-40-3) and 0.1 % of Acid red dye (CAS 3734-67-6).

[0061] The amount of polymeric coating was found to be 0.51 wt.-% (see Table 1).

Preparation of Particles B'

[0062] Particles B' were produced in the same manner as Particles A', except for using the above Coating Solution B instead of the Coating Solution A and performing the pre-drying at a temperature of 120°C instead of 150°C.

[0063] The amount of polymeric coating was found to be 0.64 wt.-%, indicating that at 120°C a crosslinking of the components may occur, thereby reducing the amount of evaporated components during the drying process (see Table 1).

[0064] Incidentally, in the present invention the amount of polymeric coating can be determined by STA using a STA 449 F3 Jupiter^®, available from Netzsch, following ISO 11357-1:2016 and 11358-1:2014 using an argon 5.0 atmosphere. Particle sizes can be determined by a laser diffraction method using e.g. a Helos Particle Size Analysis (Sympatec) following SS-ISO 13320-1.

Reference Particles

[0065] As Reference Particles, the gas atomized stainless steel particles 316L (obtainable from Carpenter under the tradename CarTech^® 316L) were used without any further treatment.

Formation of TRS Bars

[0066] TRS-Bars of 30 mm x 10 mm (length x width) and roughly 6mm high were made in a plastic mold with the specified dimensions needed for doing three-point bending tests to evaluate the Green Strength.

[0067] After disposing the particles indicated in Table 1 in bar form, the particles were preliminary bound together by manually adding a Wetting Ink A or B (binder composition) as used in a 3DP method, where the binder composition /Wetting Ink is ejected from a inkjet head on a bed of the particles to form a green part. The addition was effected with the aid of a variable volume pipette, (Finnpipette F1 Thermo Scientific). The amount of Wetting Ink for each Example and Comparative Example is indicated in Table 1, as is the composition of the Wetting Inks A and B. These correspond to the solutions to which PVP was added in the preparation of Coating Solutions A and B.

[0068] After addition of the Wetting Ink, the bars were dried at 200 °C for 3 h in an oven, and once cooled extracted from the form. Thereafter, Green Strength tests and thermal analysis (STA) for determining the TRS Bar polymer content.

[0069] STA was performed as outlined above. The Green Strength was evaluated by the following method:

Determination of Green Strength

[0070] The GS is obtained through a transversal rupture strength (TRS), three-point bend test, following ISO 3995:1985.

[0071] The specimen (TRS bar) is placed on two supports with a fixed distance, and then a from above a force is applied on the center of the specimen. The maximum force applied before sample failure is registered as the Green Strength. As the specimen (TRS bar) is rectangular, the formula shown below can be used to obtain the maximum Green Srength:

where the expressions have the following meanings (see also Figure 2) :

GS : Green Strength (MPa)

P : Force (N)

S = length between support rollers (in mm)

h : height of specimen (in mm)

b : width of the specimen (in mm) (equivalent to L in Figure 2 for square test specimen).

[0072] In order to determine the Green Strength, the dimensions of the TRS bars were measured. The TRS Bars were afterwards tested in a three-point bending machine, following ISO 3995:1985. The force signal was received by a Force transducer (TH-UM T-Hydronics Inc), and registered by a force indicator (Nobel Elektronik BKI-5).

[0073] The materials and results are summarized in the following Table 1:

Table 1

Example	Particles	Coating Solution Type	Amount of polymeric coating (wt. %)	Wetting Ink	Wetting amount (ml)	TRS Bar polymer content (wt %)	Green Strength (MPa)
Example 1	A	A	0.68	A	0.63	1.03	2.1
Example 2	A	A	0.68	A	1.26	1.29	3.9
Example 3	A	A	0.68	B	0.63	0.97	1.5
Example 4	A	A	0.68	B	1.26	1.17	4.4
Example 5	B'	B	0.64	B	1.26	1.27	3.3
Example 6	A'	A	0.51	A	1.26	1.17	3.3
Example 7	A'	A	0.51	B	1.26	1.06	3.2
Example 8	B	B	0.51	A	1.26	1.22	4.3
Example 9	B	B	0.51	B	1.26	0.82	4.3
Comparative Ex. 1	Reference	n.a.	0	A	0.63	0.19	2.7
Comparative Ex. 2	Reference	n.a.	0	A	1.26	0.41	0.7
Comparative Ex. 3	Reference	n.a.	0	B	1.26	0.43	1.8

Coating Solution Type A:
A solution prepared by adding 7.5 g of PVP (Mw=25,000) to 50 ml of a solution of 74.65% water, 12 % triethylene glycol, 5 % 1,2 hexane diol, 3.25 of nonionic surfactant, 5% of PVP with an Mw of 25,000 - 30,000 and 0.1 % of a Cyan dye

Coating Solution Type B:
A solution prepared by adding 7.5 g of PVP (Mw=25,000) to 50 ml of a solution of 84.4% water, 10 % ethylene glycol, 5% of PVP with an Mw of 15,000, 0.5 % of Tego Wet^™ 500 (nonionic surfactant, Oxirane 2-methyl-, polymer with oxirane, mono(3,5,5-trimethylhexyl) ether, CAS 204336-40-3) and 0.1 % of Acid red dye (CAS 3734-67-6)

Wetting ink A:
74.65% water, 12 % triethylene glycol, 5 % 1,2 hexane diol, 3.25 of nonionic surfactant, 5% of PVP with an Mw of 25,000 - 30,000 and 0.1 % of a Cyan dye

Wetting ink B:
84.4% water, 10 % ethylene glycol, 5% of PVP with an Mw of 15,000, 0.5 % of Tego Wet^™ 500 (nonionic surfactant, Oxirane 2-methyl-, polymer with oxirane, mono(3,5,5-trimethylhexyl) ether, CAS 204336-40-3) and 0.1 % of Acid red dye (CAS 3734-67-6)

[0074] The following can be derived from the results provided in Table 1:
Comparative Example 1 using the Reference Particles (not surface-coated) leads to a Green Strength of 2.7 MPa. This is sufficient for many applications, but is insufficient in case of thin or delicate structures, as here there is an increasing risk of deformation or breaking of the green part upon removal from the powder bed 3D-printer processing unit. Further, the particles are not protected on their surface, which may lead to surface alterations upon storage of the particles between the production and the 3D-manufacturing process.

[0075] An attempt to increase the Green Strength of the green part by increasing the amount of Wetting Ink (binder composition for forming the green part) failed. Rather, the Green Strength was lowered from 2.7 MPa in Comparative Example 1 to 1.7 MPa in Comparative Example 2. This shows that an increase of the Green Strength cannot be obtained for non-coated polymer particles by simply increasing the amount of binder/Wetting Ink. In fact, the Green Strength is significantly lower in Comparative Example 2.

[0076] The same result was also obtained when instead another type of Wetting Ink (Wetting Ink B) was used. While the Green Strength is higher as compared to Comparative Example 2, still in Comparative Example 3 the Green Strength is below the Green Strength obtained with a low amount of Wetting Ink (0.63 ml), as obtained in Comparative Example 1. Comparative Examples 2 and 3 thus show that the Green Strength of a non-coated particle cannot be increased by increasing the amount of Wetting Ink, but that instead a decrease of the Green Strength is actually obtained.

[0077] Example 1 was performed in the same manner as Comparative Example 1, except that the coated particles A were used. The amount of Wetting Ink was however identical. Example 1 led to a Green Strength of 2.1 MPa, which is sufficient for many applications, e.g. for the preparation of solid structures requiring no extremely high Green Strength.

[0078] Example 2 corresponds exactly to Example 1, except that the amount of Wetting Ink was doubled to 1.26 ml. Contrary to the result obtained in Comparative Example 2, however thereby a significant increase of the Green Strength to 3.9 MPa could be obtained. This shows that the particles of the invention are more versatile in that they allow increasing the Green Strength by simply increasing the amount of Wetting Ink, contrary to the non-coated Reference Particles.

[0079] The same trend was obtained in Example 3. Here, an amount of Wetting Ink of 0.63 ml led to a Green Strength of 1.5 MPa, which is sufficient for solid objects not requiring a high Green Strength. Again, by increasing the amount of Wetting Ink to 1.26 ml, the Green Strength could be raised to 4.4 MPa. Examples 3 and 4 thus confirm the results of Examples 1 and 2, even if the Wetting Ink A is replaced by Wetting Ink B.

[0080] Examples 5 to 9 also confirm the results obtained in Examples 1 to 4 in that an increase of the amount of Wetting Ink leads to a significant increase in Green Strength to at least 3 MPa. Although the reason for this is not clear, a comparison of Examples 4 to 9 reveals that the particles A and B generally lead to a higher Green Strength as compared to the particles A' and B'. The differences between these particles is only that the drying of the coating solution was done with a preliminary drying step at 120 or 150°C for 2 hours prior to drying at 200°C for 3 hours. It appears that a rapid drying treatment, without a preliminary drying, leads to a different surface structure, possibly due to crosslinking of the polymer in the coating, which subsequently leads to a higher Green Strength. In this respect, it is noteworthy that the Green Strength obtained in Examples 2, 4, 8 and 9 was 3.9 MPa or higher, whereas in Examples 5, 6 and 7 (using the particles A' or B') the Green Strength was in the order of 3.2 to 3.3 MPa.

EXAMPLES 10-13

[0081] Polymer Particles C and D were prepared in a similar manner as Polymer Particles A and B, except that the amount of PVP (Mw of 25,000) added to the coating solutions A/B was increased to reach a theoretical PVP content of not 1% by weight (as for Particles A and B), but of 2% by weight. The amount of 316L stainless steel particles was thus reduced to 980 g.

[0082] Additionally, the mixing process was altered by using a "Cyclomix high shear impact mixer", (Hosokava Micron B.V.). The mixer has a capacity of around 10 kg, vacuum can be applied during mixing and heating up to 150 °C of the mixing chamber is possible. The rotation speed can be varied between 60-1750 rpm.

[0083] For Particles D using coating solution B', not only the amount of PVP was increased as compared to coating solution B, but also surfactant content was increased to 2,5% (Tego Wet), and an additional surfactant was added in an amount of 2.5 % (BYK DYNWET^™ 800 N, an alcohol alkoxylate).The exact compositions of the coating solutions A' and B' are given below Table 2.

[0084] The mixing process began with introducing all of the 316L SS particles into the mixing chamber. Then, vacuum was applied, mixing started at 160 rpm and heating ramped towards 110 °C. When reaching 110 °C, the mixing and vacuum pump remained active while 20 ml of the premixed coating solution was injected every 5 minutes.

[0085] After having injected all the coating solution, the temperature was increased to 150 °C and kept at this temperature for one hour. Subsequently the vacuum and heating were discontinued and the coated particles were poured into an iron/steel crucible that was placed in an oven for drying (3 hours at 200 °C).

[0086] The obtained particles were analyzed with respect to their polymer coating content. Further, TRS test bars were prepared (green parts) and tested for their Green Strength and Polymer content, in the same manner as described above for Examples 1 - 9. The results are summarized in Table 2.

Table 2

Example	Particles	Coating Solution Type	Amount of polymeric coating (wt. %)	Wetting Ink	Wetting amount (ml)	TRS Bar polymer content (wt %)	Green Strength (MPa)
Example 10	C	A'	1.24	A	1.26	1.90	2.7
Example 11	C	A'	1.24	B	1.26	1.78	3.5
Example 12	D	B'	1.01	A	1.26	1.52	3.0
Example 13	D	B'	1.01	B	1.26	1.39	5.0

Coating Solution Type A':
A solution prepared by adding 16.25 g of PVP (Mw=25,000) to 50 ml of a solution of 74.65% water, 12 % triethylene glycol, 5 % 1,2 hexane diol, 3.25 of nonionic surfactant, 5% of PVP with an Mw of 25,000 - 30,000 and 0.1 % of a Cyan dye

Coating Solution Type B':
A solution prepared by adding 16.25 g of PVP (Mw=25,000) to 50 ml of a solution of 79.9% water, 10 % ethylene glycol, 5% of PVP with an Mw of 15,000, 2.5 % of Tego Wet^™ 500 (nonionic surfactant, Oxirane 2-methyl-, polymer with oxirane, mono(3,5,5-trimethylhexyl) ether, CAS 204336-40-3), 2.5 % of BYK DYNWET 800 N and 0.1 % of Acid red dye (CAS 3734-67-6)

Wetting ink A:
74.65% water, 12 % triethylene glycol, 5 % 1,2 hexane diol, 3.25 of nonionic surfactant, 5% of PVP with an Mw of 25,000 - 30,000 and 0.1 % of a Cyan dye

Wetting ink B:
84.4% water, 10 % ethylene glycol, 5% of PVP with an Mw of 15,000, 0.5 % of Tego Wet^™ 500 (nonionic surfactant, Oxirane 2-methyl-, polymer with oxirane, mono(3,5,5-trimethylhexyl) ether, CAS 204336-40-3) and 0.1 % of Acid red dye (CAS 3734-67-6)

[0087] Further tests were conducted in order to evaluate whether the increased amount of polymer in the green parts led to any problems or high porosity in the final object. For this test, cubes of 11 × 11 × 7 mm were printed using the 3D printer (Digital Metal P0601) from the Reference Particles using Wetting Ink A indicated in Table 2 (Reference Cube), as well as Particles C using Wetting Ink A (Cube A) and Wetting Ink B (Cube B). The green parts were thermally debound (350 °C for 180 minutes) and sintered (Temperature Profile: temperature increase to 1100°C at 3°C/minute, holding time of 15 minutes, temperature increase at 3°C/minute to 1360°C, holding time of 120 minutes, temperature decrease at 2°C/minute to 1060°C, holding time of 240 minutes, and temperature decrease at 10°C/minute to room temperature), and the relative density (as compared to bulk stainless steel) was determined.

[0088] The Reference Particles (non-coated) achieved a relative density of 97.8%. While the relative density of the test cubes A and B obtained from the particles of the invention was expectedly somewhat lower in view of the increased volume of polymer that may not fully be filled by particle core material during debinding and sintering, nonetheless very good relative densities of 97,3 % (Cube A) and 97.5 % (Cube B) were obtained.

[0089] Further, the average shrinkage during sintering was determined. For the Reference Powder, an average shrinkage of 15% was determined, while the particles of the present invention led to average shrinkages of between 18 % (Cube A) and 24 % (Cube B). This shows that the increase in Green Strength is accompanied by only small to moderate increases in the shrinkage and small to moderate reductions in the relative density.

Claims

1. Particles each having a sinterable core made of stainless steel and a polymeric coating on at least a part of the core, wherein the polymeric coating contains 80% by weight or more relative to the total weight of the polymeric coating of polyvinyl pyrrolidone having a weight average molecular weight (Mw) of from 1,000 - 50,000 g/mol measured according to a GPC-method using polystyrene as standard, that can be removed via decomposition by heat, catalytically or by solvent treatment, and wherein the polymeric coating is present in an amount of 0.10 to 10.00 % by weight, relative to the total weight of the particles.

2. Particles according to claim 1, wherein the polymeric coating comprises a surfactant, preferably a nonionic surfactant, and/or a wetting agent, which preferably comprises a polyol.

3. Particles according to any one of claims 1 or 2 having a particle size distribution such that 95% by weight or more of the particles have a diameter of 50 µm or less (X₉₅ ≤ 50 µm) measured by laser diffraction according to SS-ISO13320-1.

4. Use of the particles as defined in any one of claims 1 to 3 in an additive manufacturing process, in particular a powder bed and inkjet-head 3D printing process.

5. Use according to claim 4, wherein a liquid binder composition is used for preliminary binding of the particles, the liquid binder composition comprising polyvinyl pyrrolidone, and wherein the same polymer type is present in the polymeric coating of the particles, each polymer of the same type having a Mw in the range of 1,000 - 50,000 as measured according to a GPC-method using polystyrene as standard.

6. A green part obtainable by binding the particles as defined in any one of claims 1 to 3 with a binder composition.

7. The green part according to claim 6, wherein the binder comprises polyvinyl pyrrolidone, and wherein the same polymer type is present in the polymeric coating of the particles, each polymer of the same type having a Mw in the range of 1,000 - 50,000 as measured according to a GPC-method using polystyrene as standard.

8. The green part according to claim 6 or claim 7, which has a green strength of 2.7 MPa or more, preferably 3.0 MPa or more.

9. An additive manufacturing method, comprising a step of binding the particles as defined in any one of claims 1 to 3 with a binder composition to form a green part, a step of removing the binder composition and the polymeric coating by heat treatment, solvent extraction or catalytically to form a brown part, and a step of sintering the brown part to obtain an object made from a material of the core of the particles.

10. Additive manufacturing method according to claim 9, which is a powder bed and inkjet head 3D printing method.

Ansprüche

1. Partikel, jeweils aufweisend einen sinterfähigen Kern, der aus Edelstahl hergestellt ist und eine Polymerbeschichtung auf zumindest einem Teil des Kerns, wobei die Polymerbeschichtung 80 Gew.-% oder mehr relativ zu dem Gesamtgewicht der Polymerbeschichtung von Polyvinylpyrrolidon enthält, das ein gewichtsmittleres Molekulargewicht (Mw) von 1.000 - 50.000 g/mol gemessen gemäß einem GPC-Verfahren unter Verwendung von Polystyrol als Standard aufweist, die durch Zersetzung durch Wärme, katalytisch oder durch Lösungsmittelbehandlung entfernt werden kann, und wobei die Polymerbeschichtung in einer Menge von 0,10 bis 10,00 Gew.-% relativ zu dem Gesamtgewicht der Partikel vorhanden ist.

2. Partikel nach Anspruch 1, wobei die Polymerbeschichtung ein Tensid, bevorzugt ein nichtionisches Tensid, und/oder ein Netzmittel, das bevorzugt ein Polyol umfasst, umfasst.

3. Partikel nach einem der Ansprüche 1 oder 2, aufweisend eine Partikelgrößenverteilung, sodass 95 Gew.-% oder mehr der Partikel einen Durchmesser von 50 µm oder weniger (X₉₅ ≤ 50 µm) gemessen durch Laserbeugung gemäß SS-ISO13320-1 aufweisen.

4. Verwendung der Partikel nach einem der Ansprüche 1 bis 3 in einem additiven Fertigungsprozess, insbesondere einem Pulverbett- und Tintenstrahlkopf 3D-Druckprozess.

5. Verwendung nach Anspruch 4, wobei eine flüssige Bindemittelzusammensetzung zum vorläufigen Binden der Partikel verwendet wird, wobei die flüssige Bindemittelzusammensetzung Polyvinylpyrrolidon umfasst, und wobei der gleiche Polymertyp in der Polymerbeschichtung der Partikel vorhanden ist, wobei jedes Polymer des gleichen Typs ein Mw in dem Bereich von 1.000 - 50.000 wie gemessen gemäß einem GPC-Verfahren unter Verwendung von Polystyrol als Standard aufweist.

6. Grünling, erhältlich durch Binden der Partikel nach einem der Ansprüche 1 bis 3 mit einer Bindemittelzusammensetzung.

7. Grünling nach Anspruch 6, wobei das Bindemittel Polyvinylpyrrolidon umfasst und wobei der gleiche Polymertyp in der Polymerbeschichtung der Partikel vorhanden ist, wobei jedes Polymer des gleichen Typs ein Mw in dem Bereich von 1.000 - 50.000 wie gemessen gemäß einem GPC-Verfahren unter Verwendung von Polystyrol als Standard aufweist.

8. Grünling nach Anspruch 6 oder Anspruch 7, der eine Grünfestigkeit von 2,7 MPa oder mehr, bevorzugt 3,0 MPa oder mehr aufweist.

9. Additives Fertigungsverfahren, umfassend einen Schritt des Bindens der Partikel nach einem der Ansprüche 1 bis 3 mit einer Bindemittelzusammensetzung, um einen Grünling zu bilden, einen Schritt des Entfernens der Bindemittelzusammensetzung und der Polymerbeschichtung durch Wärmebehandlung, Lösungsmittelextraktion oder katalytisch, um einen Bräunling zu bilden, und einen Schritt des Sinterns des Bräunlings, um einen Gegenstand zu erhalten, der aus einem Material des Kerns der Partikel hergestellt ist.

10. Additives Fertigungsverfahren nach Anspruch 9, das ein Pulverbett- und Tintenstrahlkopf-3D-Druckverfahren ist.

Revendications

1. Particules ayant chacune un noyau frittable en acier inoxydable et un revêtement polymère sur au moins une partie du noyau, où le revêtement polymère contient 80 % en poids ou plus par rapport au poids total du revêtement polymère de polyvinylpyrrolidone ayant un poids moléculaire moyen en poids (Mw) de 1 000 à 50 000 g/mol mesuré selon un procédé GPC utilisant le polystyrène comme étalon, qui peut être éliminé par décomposition par traitement thermique, catalytique ou au solvant, et où le revêtement polymère est présent en une quantité de 0,10 à 10,00 % en poids, par rapport au poids total des particules.

2. Particules selon la revendication 1, où le revêtement polymère comprend un tensioactif, de préférence un tensioactif non ionique, et/ou un agent mouillant, comprenant de préférence un polyol.

3. Particules selon l'une quelconque des revendications 1 ou 2 ayant une distribution granulométrique telle que 95 % en poids ou plus des particules ont un diamètre de 50 µm ou moins (X₉₅ ≤ 50 µm) mesuré par diffraction laser selon SS-ISO13320-1.

4. Utilisation des particules telles que définies dans l'une quelconque des revendications 1 à 3 dans un procédé de fabrication additive, en particulier un procédé d'impression 3D à tête à jet d'encre et sur lit de poudre.

5. Utilisation selon la revendication 4, dans laquelle une composition de liant liquide est utilisée pour la liaison préliminaire des particules, la composition de liant liquide comprenant de la polyvinylpyrrolidone, et dans laquelle le même type de polymère est présent dans le revêtement polymère des particules, chaque polymère du même type ayant un Mw compris entre 1 000 et 50 000 mesuré selon un procédé GPC utilisant le polystyrène comme étalon.

6. Une pièce verte obtenue par liaison des particules telles que définies dans l'une quelconque des revendications 1 à 3 avec une composition de liant.

7. La pièce verte selon la revendication 6, dans laquelle le liant comprend de la polyvinylpyrrolidone, et dans laquelle le même type de polymère est présent dans le revêtement polymère des particules, chaque polymère du même type ayant un Mw compris entre 1 000 et 50 000 mesuré selon un procédé GPC utilisant le polystyrène comme étalon.

8. La pièce verte selon la revendication 6 ou 7, qui présente une résistance à cru de 2,7 MPa ou plus, de préférence 3,0 MPa ou plus.

9. Procédé de fabrication additive, comprenant une étape de liaison des particules telles que définies dans l'une quelconque des revendications 1 à 3 avec une composition de liant pour former une pièce verte, une étape d'élimination de la composition de liant et du revêtement polymère par traitement thermique, extraction par solvant ou catalyse pour former une pièce brune, et une étape de frittage de la pièce brune pour obtenir un objet constitué d'un matériau du noyau des particules.

10. Procédé de fabrication additive selon la revendication 9, qui est un procédé d'impression 3D à tête à jet d'encre et sur lit de poudre.

Drawing